Process for extraction and recovery of aromatic hydrocarbons from hydrocarbon mixtures



.lune 2l, 1955 N` PoFFENBERGr-:R 2,711,433

' PROCESS FOR EXTRACTION AND RECOVERY OF AROMTIC Filed June 2, 1952 l N VEN TOR AKO/and Pof/ne/yer @QCQQQQ ATTORNEYS PROCESS FOR EXTRACTN AND RECOVERY 0F AROMATIC HYDROCARBONS FRGM HYDRO- CARBON MiXTURES Noland Poenberger, Midland, Mich., assigner to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application June 2, 1952, Serial No. 291,192

5 Claims. (Cl. 260-674) This invention relates to an improved process for the selective solvent extraction and recovery of aromatic hydrocarbons from mixtures thereof with non-aromatic hydrocarbons. lt relates in particular to a method whereby the separation and recovery of individual aromatic hydrocarbons of industrial grade or better is made possible without need for chemical treatment.

Several processes are known for the selective extraction of aromatics from mixed hydrocarbon feeds such, for example, as the reformed naphthas. In general, such processes comprise countercurrent liquid-liquid contact between the mixed hydrocarbon feed and any of several selective solvents, with the extract phase, containing most of the aromatics, being withdrawn from one end of the system while a railinate phase, containing little or no solvent little or no aromatic hydrocarbon, is withdrawn from the other end of the extraction system. It is found' that, the greater the carrying power of the selective solvent for aromatic hydrocarbons, the greater will be the amount of non-aromatics which leaves the extraction system with the extract phase. lt is also noted that when the feed and the co-extracted non-aromatics contain considerable unsaturates it is diflicult to separate such unsaturates from the aromatic hydrocarbons and, accordingly, the latter seldom meet the established standards for industrial or better grades of aromatics without a chemical treatment.

lt is accordingly among the objects of the present invention to provide an improvement in processes for the selective solvent extraction of aromatic hydrocarbons from mixtures thereof with non-aromatics, some of which may be unsaturated, which will lead to the recovery of individual aromatics of industrial grades, or better, without necessity for a chemical after-treatment. A related object is to provide such an improved process in which the fractions cf an aromatic-rich extract from such a mixed feed which normally cause difculty in separation of the aromatics may be removed preferentially from the extract and return to the extraction system for displacement in large part to the non-aromatic ranate. is to provide such a method in which the troublesome light ends from the aromatic-rich extract are separated azeotropically from the balance of the extract and are returned to the extraction system. Other and related objects rnay appear hereinafter.

in accordance with the present invention, a mixed hydrocarbon feed, containing aromatic hydrocarbons and non-aromatic hydrocarbons, is supplied as a liquid phase to a countercurrent multistage extraction system near the middle, and a liquid which is a preferential solvent for aromatic hydrocarbons but which is substantially imniiscible with non-aromatic hydrocarbons is supplied to the system near one end. A substantially aromatic-free and solvent-free raffinate is withdrawn from the same end and an aromatic-rich extract is withdrawn from the other end. The extract is prestripped to remove therefrom as vapors substantially all of the hydrocarbons which are more volatile than benzene, together with part of the Another object benzene. The partially stripped extract is then fully stripped to recover the remaining hydrocarbons therefrom, the solvent is returned to the extraction system and the hydrocarbons are rectified. Any light ends from the rectification, boiling below benzene, are conveyed to a column to which the hydrocarbon vapors from the prestripping unit are also conveyed and in which the severalV hydrocarbons are admixed with an agent which forms azeotropes with at least part of the non-aromatic hydrocarbons. The resulting azeotropic fractions are taken overhead from said column and are condensed and re- `turned to the extraction system at a point intermediate between the point at which the original mixed feed is introduced and the end from which the aromatic-rich extract is withdrawn, preferably at the point in the extractor where the hydrocarbons have the same analysis as those in the azeotrope. The aromatic-rich bottoms from the column in which the azeotrope is distilled are returned to the extraction system as reflux near the end from which the4 extract is withdrawn. The return of the non-aromatics has the effect of increasing the relative proportion of these materials in the feed, and, when equilibrium conditions are reached, substantially all of the non-aromatics in the feed are found in the raiinate. Any water returned with the azeotrope simply dissolves in the solvent with the usual effect of increasing its selectivity. The return of the light aromatics has the effect of increasing the carrying power of the solvent for aromatics and, when equilibrium conditions are reached, substantially all of the aromatics which enter the extractor with the feed are found in the stream of aromatics being sent to the rectitier. The azeotroping agent usually exerts little effect upon the solvent power of the extractant, but aids materially in separation of lightends from the extract.

The extraction system is operated under such pressure` as may be needed to maintain the entire contents in the liquid condition at the temperature employed. Extraction is commonly effected at somewhat elevated temperatures, of the order of 50 to 150 C., and when the extract is removed at such temperature and released to a lower pressure in the prestripper, the light ends tend to flash away from the extract without supplying much, if any, additional heat thereto. Similarly, the low boiling azeotropes tend to ash from the azeotrope still with the addition of minimum quantities of heat, so that the process represents a significant heat economy. The heat supplied in the final stripper raises the solvent to the required temperature for use in the extraction system, and this same heat performs the work needed in the prestripper and the azeotrope still before it is lost to condensers.

The solvent employed may be any of the selective solvents for aromatics. Such solvents include the polyglycols, phenols, glycol monoethers, glycol monoesters, various alcohols, alkanolamines, amines and amides. Of these, the polyglycols are preferred. This group includes diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol and mixtures of these with one another and often with some water.

The commonly occurring non-aromatics in an aromatized hydrocarbon feed, which may follow the aromatics into the extract, include such saturates as the pentanes, cyclohexane, methylcyclohexane, n-hexane, 3-methylpentane, and Z-methylpentane, and such typical unsaturates as pentenes, hexenes, methylcyclopentene and cyclohexene, as well as some hexadienes and cyclohexa'diene. Occasionally some compounds of similar molecular size are found having acetylenic unsaturation (triple bonds).

The azeotroping agents which may be employed are any of a large group of compounds, non-reactive under prevailing conditions with the selective solvent employed, which are known to form low-boiling binary azeotropes with the lighter hydrocarbons present in the mixed feed.

Especially desirable are those agents which form azeotropes with the non-aromatic hydrocarbons present but which do not do so with benzene. Numerous useful compounds are listed in a Table of Azeotropes and Nonazeotropes, by L. H. Horsley, Analytical Chemistry, vol. 19, pages 508-609 (1947) and vol. 21, pages 831-873 (1949). The useful compounds include, but are not limited to, acetone, acetonitrile, allyl alcohol, Z-butanone, butyl alcohol, isobutyl alcohol, sec-butyl alcohol, tert-butyl alcohol, butyraldehyde, isobutyraldehyde, ethanol, ethyl acetate, ethyl formate, ethyl nitrate, methanol, methyl formate, methyl propionate, nitroethane, nitromethane, l-propanol, 2-propanol, propylene oxide, propyl formate and pyridine. The preferred azeotroping agent for use in the present process is acetone, which does not form an azeotrope with benzene, but does so with most of the non-aromatic hydrocarbons having 5 to 7 carbon atoms. In some cases, there is no objection to the azeotroping agent forming an azeotrope with benzene as well as with the non-aromatics. Methanol, ethanol, 1propanol and 2-propanol are among the most satisfactory agents in such a case.

The new process will be described with reference to the annexed drawing, the single figure or" which is a flow diagram of the process.

A mixed aromatic and non-aromatic hydrocarbon feed is introduced through line into a countercurrent multistage extraction system 11 near the middle. A liquid selective solvent for aromatic hydrocarbons is introduced into extractor 11 near one end, and a rainate, substantially free from aromatics and from the selective solvent, is withdrawn from the same end through line 12. An extract, containing aromatic hydrocarbons dissolved in the solvent is withdrawn from the other end of extractor 11 through line 13 and is conveyed to a prestripping still 14 from which the hydrocarbons which are more volatile then benzene, and at least part of the benzene, are distilled through overhead line 15 and conveyed to an azeotrope still 16. 'Ille partially stripped extract is withdrawn as bottoms from still 14 through line 17 to a iinal stripper 18, in which enough heat is supplied to free the solvent of its hydrocarbon content. The stripped solvent is withdrawn as bottoms from the heated iinal stripper 18 through line 19 and is returned to extractor 11. The hydrocarbon vapors passing overhead from stripper 1S are condensed by condenser 20 and the condensate ilows to a fractionating column 21 through line 22. Any non-aromatic light ends of the hydrocarbons fed to column 21 are taken overhead, condensed, and passed through line Z3 to the azeotrope still 16. The aromatic hydrocarbons are taken as bottoms from column 21 and are sent to a rectifying system for recovery of individual aromatics. The light endsV from prcstripper ld and those from column 21 are mixed in the azeotrope still 1e with an agent which forms azeotropes of low boiling point at least, and preferably only, with the light non-aromatic hydrocarbons present. The azeotropes pass overhead through condenser 24, from which part of the condensate is returned through line 2S as reux to still 14 and the balance is returned through line 26 to the extraction system 11, entering at a point intermediate between the point of entry of the mixed feed and the point of withdrawal of the aromatic-rich extract. The azeotroping agent is admitted as required to still 16 through line 27 from a supply vessel 28 to maintain the desired analysis at the head of still 16. The bottoms from still 16, consisting chieiiy of benzene, the light hydrocarbons which do not azeotrope with the agent employed and are less voltatile than the azeotropes, and possibly small amounts of the azeotroping agent, are returned through line 29 to extractor 11, entering near the end from which the aromatic-rich extract is withdrawn.

The azeotroping agent ows through that part of the system which is shaded in the annexed drawing. The

amount of azeotroping agent employed is normally very small. Thus, since the carrying power of the solvent for aromatics is most commonly from 10 to 20 per cent, there will ordinarily be considerably less than 1 per cent or non-aromatics in the extract entering the prestripper 14. ln most cases, the amount of azeotroping agent needed to form an azeotrope with the hydrocarbons does not exceed the weight and is often as little as l0 to 1S per cent of the weight of the hydrocarbons. Hence, in practical operations, the amount of azeotroping agent needed ranges from about 0.1 to 1 per cent of the weight of solvent being circulated to the extractor, though the use of somewhat larger proportions is not harmful to the process and the use of smaller amounts will still be advantageous in reducing the amount of non-aromatics moving with the aromatics toward the rectifying system. Approaching the problem from another direction, the usual solvent extraction system forms an extract in which the hydrocarbons consist of at least 98 per cent aromatics and 2 per cent or less of non-aromatics. If 4 parts by weight or the extracted hydrocarbons are returned to the extractor as reflux for each part by weight of purified aromatic product, the impurities in the initial extract represent 0.1 part by weight for each part of purified product being recovered at the end of the train. The amount of azeotroping agent required is then from 0.01 to 0.1 pound for each pound of recovered aromatic hydrocarbon.

The azeotroping agents useful in the process have in common the property of dissolving in the aromatic-rich extract phase leaving the extractor to a sutiicient extent` so that the large tiow of solvent relative to the amount of azeotroping agent efectively removes that agent from the extractor and prevents it from being lost to the rafiinate phase. The prestripper 14 and final stripper 1S assure the absence of azeotroping agent from the lean solvent returned through line 19 to the extractor 11.

To illustrate the diiiiculty of separating a pure aromatic hydrocarbon from its mixture with unsaturated nonaromatics by simple recticaton, the following example is given:

A reformed naphtha having a boiling range from 140 to 320 F. was subjected to careful fractionation. The original mixture had a bromine index of 2660 (Analytical Chemistry, vol. 19, page 869 (1947)). Bromine index is the volume, in milliliters, of 0.01 normal bromatebromide reagent which will react with the unsaturates in a 100 milliliter sample of the hydrocarbon being tested. Careful rectification showed the following even distribution of unsaturates in all fractions boiling below 230 F. C.).

Boilinrr Range Weight Fraction Vv elght, o C at percent Bromme index 760 lm Aromatics 18 29. 5-50. 5 0 N ot measured. 23 50. 5-61. 5 0 Do. 66 61. 5-70. 0 0 1950. 53 70. 0-81. 0 39. 0 2720. 21 81. 0-81. 0 74. 5 1820. 9 81. 0-91. 0 18. 0 2250. 38 91. 0-94. 0 0. 7 1780. 50 94. 0-110 0 26. 5 2970.

T5 and the recovered hydrocarbon portion of the extract was rectiied. Results on the portions boiling up to 110 C. are given below:

1701 N ot measured.

sesam...

SSSE'.

ses

CIOC

aaaaase Total It is noted that the distribution of unsaturation has been altered by the extraction process but that significant amounts of unsaturates are still found in the products boiling at the proper temperature for benzene.

The same feed was extracted at 70 C. in the same manner with volumes of diethylene glycol per volume of feed, and with 4 pounds of benzene from the aromatic stream being returned as reflux to the extractor for each pound of aromatics being recovered from the rectifier, and 0.1 pound of acetone was supplied to the azeotrope still for each pound of aromatics being recovered. The light ends were almost completely removed by the acetone and only very small amounts of light hydrocarbons were found in the aromatic stream entering fractionating tower 21. The azeotropes of acetone and light hydrocarbons were returned to the extractor, as illustrated in the drawing, and the aromatic bottoms from still 16, returned as reux to the extractor, consisted essentially of benzene. The portion of the final aromatic stream which boiled up to 110 C. had the following analysis:

The benzene recovered from the finishing stills had a freezing point of 5.3 to 5.5 C., a boiling range of less than 2 C., and in all respects met the specifications for industrial grade benzene (A. S. T. M.-specifcation: D836-47) without further chemical treatment. The higher aromatics present were also obtained in a degree of purity meeting their respective standards for industrial grades.

Similarly advantageous results are obtained when using methanol, ethanol, l-propanol, 2-propanol, or a butyl alcohol, or propylene oxide, ethyl formate, nitromethane, or one of the other previously named azeotroping agents in place of the acetone of the foregoing example. Since most of these form azeotropes with benzene as well as with the light non-aromatics from the extract, there is less sharp separation of aromatics from non-aromatics in the azeotrope still, but the return of the azeotropes to the extractor near the end thereof from which the aromaticrich extract is removed results in the same favorable shift in the equilibrium conditions, displacing more nonaromatics to the rainate and yielding a purer aromatic stream to the recters.

I claim:

1. In a process in which a mixture of aromatic and non-aromatic hydrocarbons is subjected to countercurrent multistage extraction in the liquid phase with a selective solvent for aromatic hydrocarbons and an extract rich in aromatic hydrocarbons is withdrawn from one end of the extraction system, the extract is stripped of its hydrocarbon content and the stripped solvent is returned to the other end of the extraction system, the improvement which consists in: effecting at least part of the said stripping operation in the presence of a small but effective amount of an agent which is non-reactive with the solvent and is capable of forming low-boiling azeotropes with at least part of the non-aromatic hydrocarbon portion of the extract but does not form an azeotrope with benzene, separating the azeotropes and benzene from the extract and from one another, and returning the azeotropes and at least part of the extracted benzene separately to the extraction system nearer the end thereof from which the extract is withdrawn than the end to which the solvent is introduced, the azeotropes being returned thereto at a point nearer the middle of the system than at which the benzene is returned.

2. The process claimed in claim 1, wherein the azeotroping agent employed is acetone.

3. In a process in which a mixture of aromatic and non-aromatic hydrocarbons is subjected to countercurrent multistage extraction in the liquid phase with a selective solvent for aromatic hydrocarbons and an extract rich in aromatic hydrocarbons is withdrawn from one end of the extraction system, the extract is stripped of its hydrocarbon content and the stripped solvent is returned to the other end of the extraction system, the improvement which consists in: supplying tothe extraction system, in amount from 0.1 to 1 per cent of the weight of solvent, an agent which is non-reactive with the solvent and which forms low-boiling azeotropes at least with part of the light non-aromatic portion of the extract; boiling from Vsaid extract in a prestripping operation the hydrocarbons therein which Vare more volatile than benzene together with part of the benzene, and together with said azeotrope-forming agent; separating the low-boiling azeotropes by distillation from higher boiling portions of said hydrocarbons; returning said higher boiling portions to the extraction system near the end from which the extract is withdrawn while returning the azeotropes to the extraction system at a point between said end and the middle; and recovering aromatic hydrocarbons from that portion of the extract remaining after the prestripping operation.

4. The process claimed in claim 3, wherein the azeo-,.

troping agent employed is one which forms azeotropes with at least part of the non-aromatic hydrocarbon portion of the extract but does not form an azeotrope with benzene.

5. The process claimed in claim 4, wherein the azeotroping agent is acetone.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,392 Tijmstra Dec. 6, 1938 2,161,567 Gee et al. June 6, 1939 2,407,820 Durrum Sept. 17, 1946 2,459,403 Ahrens Jan. 18, 1949 2,463,479 Denton et al. Mar. l, 1949 

1. IN A PROCESS IN WHICH A MIXTURE OF AROMATIC AND NON-AROMATIC HYDROCARBONS IS SUBJECTED TO COUNTERCURRENT MULTISTAGE EXTRACTION IN THE LIQUID PHASE WITH A SELECTIVE SOLVENT FOR AROMATIC HYDROCARBONS AND AN EXTRACT RICH IN AROMATIC HYDROCARBON IS WITHDRAWN FROM ONE END OF THE EXTRACTION SYSTEM, THE EXTRACT IS STRIPPED OF ITS HYDROCARBON CONTENT AND THE STRIPPED SOLVENT IS RETURNED TO THE OTHER END OF THE EXTRACTION SYSTEM, THE IMPROVEMENT WHICH CONSISTS IN: EFFECTING AT LEAST PART OF THE SAID STRIPPING OPERATION IN THE PRESENCE OF A SMALL BUT EFFECTIVE AMOUNT OF AN AGENT WHICH IS NON-REACTIVE WITH THE SOLVENT AND IS CAPABLE OF FORMING LOW-BOILING AZEOTROPES 